1. Field of the Invention
The present invention relates to improvements in a tandem master cylinder of a braking system for automobiles in which two mutually independent brake fluid pressures are produced.
2. Description of the Related Art
In a certain type of conventional tandem master cylinder, e.g., a tandem master cylinder disclosed in Japanese Patent Publication No. Sho 60-21098, the mounting load of a primary return spring located at the back of a floating piston is set greater than the mounting load of a secondary return spring located at the front of the floating piston, so that a main piston and the floating piston may be moved forward simultaneously at the start of brake application to close two return ports simultaneously, thereby decreasing an ineffective stroke at the time of braking. A small-diameter piston receives a fluid pressure from a primary fluid chamber behind the floating piston and from a secondary fluid chamber before the floating piston is inserted in a small-diameter cylinder formed in the floating piston while the main piston and the floating piston are moving forward together as one unit, the small-diameter piston is moved backward in relation to the forward movement of the floating piston by a fluid pressure built up in the secondary fluid chamber or by a rod member, thus restricting a sudden increase in the fluid pressure in the secondary fluid pressure chamber and consequently preventing biting of a piston seal of the floating piston passing over the return port. This type of tandem master cylinders, where the small-diameter piston is moved backward with the forward movement of the piston by a rod member has been disclosed in Japanese Patent Publication No. Hei 5-25705, which is as shown in FIG. 4.
In FIG. 4, of the means for restraining a sudden increase in the fluid pressure in a secondary fluid pressure chamber 53 by offsetting a relative volumetric change resulting from the backward movement of a small-diameter piston 56 which is inserted in a small-diameter cylinder 55 provided in a floating piston 54. During forward movement of the floating piston 54 when the operation of the tandem master cylinder 51 starts, a rigid rod member 57 prevents the follow-up movement of the small-diameter piston 56 in the forward direction of the floating piston 54. Therefore the positional relationship between the rear boss 58 of the small-diameter piston 56 and the head section 60 of the stopping rod 59 is restricted in design in an attempt to satisfy the overall discharge of the tandem master cylinder 51. In FIG. 4, therefore, there must be established the relationship Z=X+Y, where X is a distance between the front end of the main piston 61 in the home position and a cup-shaped spring retainer 62 (equivalent to the discharge of the primary fluid pressure chamber 52), Y is a distance between the front end 63 of the floating piston 54 in home position and the front end wall 64 of the cylinder (equivalent to the discharge of the secondary fluid pressure chamber 53), and Z is a distance between the head section 60 of the stopping rod 59 and the rear boss 58 of the small-diameter piston 56.
In the tandem master cylinder of FIG. 4, when braking operation is done without any fluid pressure built up in the primary fluid pressure chamber 52 for some reason or other, the floating piston 54 starts moving, and after the piston seal 65 has closed the return port 66, the fluid pressure is built up in the secondary fluid pressure chamber 53. At this time, a fluid pressure increase in the secondary fluid pressure chamber 53 is delayed, and accordingly there occurs a delay of brake pressure application to the braking equipment, until the small-diameter piston 56 inserted in the floating piston 54 makes a relative backward movement with respect to the floating piston 54 and the head section 60 of the stopping rod 59 which is moving forward comes into contact with the rear boss 58 of the small-diameter piston 56. A volumetric increase occurs corresponding to the volumetric decrease of the secondary fluid pressure chamber 53 caused by the forward movement of the floating piston 54. Reference numeral 67 denotes a push rod.